helper/command: make command_run_line reentrant

helper/command: make command_run_line reentrant

The `command_run_line` function contains a comment saying it should be
reentrant. However, it isn’t: it NULLs out `current_target_override` and
doesn’t restore it before returning, and it changes the `context`
associated data of the `interp` object and then deletes that associated
data before returning rather than restoring it to its previous value.

Change-Id: I84fd46ef7173f08cf7c57b9a5b76e4986a60816f
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/5223
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>